Toolbox hp-ux-lI'd suggest that you use a product called 'Big Brother'.
It's available online at http://bb4.com/ - and is a wonderful tool that will help you monitor and alert you of all types of events, 24/7.
It's easy to configure, and a real good tool, in my humble opinion.

It does a lot more than just memory usage - give it a try, and I hope you find it as valuable as I do.